It is now common practice to explore the oceans of the earth for deposits of oil, gas and other valuable minerals by seismic techniques in which an exploration vessel imparts an acoustic wave into the water, typically by use of a compressed air "gun." The acoustic wave travels downwardly into the sea bed and is reflected at the interfaces between layers of materials having varying acoustic impedances. The wave travels back upwardly where it is detected by microphone or "hydrophone" elements in a streamer towed by the vessel to yield information regarding characteristics of the underwater material and structures.
A towed streamer comprises a plurality of pressure-sensitive hydrophone elements enclosed within a waterproof jacket and electrically coupled to recording equipment onboard the vessel. Each hydrophone element within the streamer is designed to convert the mechanical energy present in pressure variations surrounding the hydrophone element into electrical signals. Due to its often extreme length (on the order of kilometers), the streamer is divided into a number of separate sections or "modules" that can be decoupled from one another and that are individually waterproof. Individual streamers can be towed in parallel through the use of paravanes to create a two dimensional array of hydrophone elements. Data buses running through each of the modules in the streamer carry the signals from the hydrophone elements to the recording equipment (so-called "acoustic data").
The individual hydrophone elements are preferably electrically coupled to one another to act in concert as a single sensor distributed along the length of each module. The individual signals of the hydrophone elements in the group are combined to produce a single group signal.
The single group signal may be thought of as emanating from a single, hypothetical hydrophone unit placed in the midst of the group of hydrophones. The location of the hypothetical hydrophone is called the "group center" of the group. Thus, when one speaks of a hydrophone array, one is often actually referring to an array of hypothetical hydrophones existing at group centers of actual groups of individual hydrophone elements.
The placement of each hydrophone in a group determines the location of each group center. It is important, for data analysis purposes, that the group centers be regularly spaced. The prior art has assumed that the individual hydrophones must likewise be regularly spaced. Occasionally, this results in a hydrophone being placed proximate a source of acoustic interference that interferes with operation of the hydrophone element and, thus, the group as a whole. It is very important that interference be reduced as much as possible, because pressure variations carrying actual acoustic data are minute. It is therefore desirable overall to reduce interference to the hydrophone elements in a module. Interference is a special concern in reduced diameter (on the order of 2") streamers.
Accordingly, what is needed in the art are construction practices for hydrophone modules that decrease the amount of interference that the hydrophones therein experience. More specifically, there is a need in the art for techniques for relocating individual hydrophones and otherwise decreasing noise in a hydrophone module.